1. Field of the Invention
The present invention relates to a switching apparatus, and more particularly to a simple and inexpensive optical switching apparatus which can function as a field disturbance sensor in a number of different types of instruments.
2. Description of the Prior Art
Optical switching devices have been known for some time. Very broadly, these may be divided into two categories. The first category includes devices having a light source and a photo-responsive element. These devices are commonly part of a system for automatically opening doors, emitting an audible signal when a door is opened manually and as a security device. The second category includes devices which are passive in that there is no artificial light source. Rather, the system is actuated by changes in the ambient light. An advantage in such a system is that it uses less power and may be more easily secreted. Such a system is ideal for security purposes. In general, the passive optical systems include two photo-responsive devices placed side by side so as to have almost identical fields of view. That is, the ambient light which is within the field "viewed" by the photo-responsive element is nearly identical for each of the two elements since they are placed in the same plane within close proximity to one another. While such a system is desirable in certain applications it has its limitations. For example, if a relatively small region was to be monitored at a distance from the photo-responsive elements, the side by side passive optical system would not work.